Epigenetic chromatin modifications such as histone methylation and acetylation are emerging as prominent regulators of human development, disease and aging. Although epigenetic modifications have been shown to be critical for neuronal function, the roles that such modifications play in maintenance of the nervous system and their contribution to neurodegenerative diseases remain largely unknown. To address this fundamental question, we recently performed an in-vivo RNAi screen for epigenetic modifiers of neurodegeneration in Drosophila, a powerful genetically amendable animal model. We began our analysis with an established model of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). These devastating diseases are characterized by the abnormal cytoplasmic accumulation of TDP-43, an RNA and DNA binding protein. Mutations in the TDP-43 coding gene (TARDBP) are found in rare forms of FTD and ALS, providing additional genetic evidence for its causal involvement. Thus, expression of TDP-43 in the Drosophila nervous system provides a model for TDP-43 toxicity associated with FTD/ALS. By knocking down the expression of over 80 genes involved in various aspects of histone modifications and chromatin remodeling, we found that several factors, all of which impact a single modification (trimethylation of histone 3 at lysine 4 (H3K4me3)), robustly modulate toxicity of TDP-43 in Drosophila, but not that of other neurodegeneration-related proteins. These findings and subsequent experiments established that reducing the global levels of H3K4me3 enhances TDP-43 toxicity, whereas increasing H3K4me3 levels mitigates TDP-43 toxicity. Launching from this discovery, we will use Drosophila and human patient tissue to define the mechanisms by which this histone methylation mark affects FTD and ALS disease progression. First, rigorous controls in Drosophila will be performed, and the effects of histone methylation on toxicity of oter FTD- and ALS-associated genes and mutations will be examined. Second, using a recently developed in- vivo, cell type-specific chromatin immunoprecipitation and deep-sequencing (ChIP-Seq) technique, chromatin landscape alterations induced by TDP-43 will be characterized. Co-IP assays will determine if any factor known to bind or affect H3K4me3, physically interacts with TDP-43. Third, the human disease relevance of these findings will be determined using post-mortem brain samples from FTD and ALS patients and controls. This combination of in-vivo studies, cell-type specific analyses and next generation chromatin investigations in animal models and human disease samples will provide an exceptional opportunity to uncover the epigenetic mechanisms underlying these incurable diseases. Our promising preliminary findings demonstrate the timely nature of the proposed study and indicate that this work will provide fundamental insight into the biology of neurodegenerative diseases and the foundation for new therapeutic approaches.